One of the oldest sports riddles is:
 
Q: What has eighteen legs and catches flies?
A: A baseball team.
 
The nine defensive players on the baseball field (18 legs) can catch flies (fly balls). The popular riddle has been cited in print since at least 1926. “What has four legs and flies?” is a similar and older riddle.
 
“What has 22 legs and 2 wings, but can’t fly?”/“A football team” is a similar sports riddle.
   
 
8 November 1926, Atchison (KS) Daily Globe, pg. 12, col. 2:
Al Weatherford: “What is it that walks around on 18 legs and catches flies? A baseball team. Ha ha ha.”
 
21 April 1928, The Evening Tribune (San Diego, CA) “Uncle Ray’s Corner,” pg. 7, col. 6:
What has 18 legs and catches flies?
 
31 January 1932, Sunday American-Statesman (Austin, TX), “College Humor,” pg. A7, col. 6:
Ump: What’s got eighteen legs and catches flies?
Ref.: A baseball team.
—Reserve Red Cat
   
Google Books
December 1936, Boys’ Life, “Think and Grin” edited by Frank J. Rigney, pg. 37, col. 2:
A Fast One
TENDERFOOT: What has eighteen legs and catches flies?
FIRST CLASS SCOUT: I don’t know, what?
TENDERFOOT: A baseball team.
 
Google Books
February 1942, Boys’ Life, “Think and Grin” edited by Frank Rigney, pg. 50, col. 3:
Tim: What has eighteen legs and catches flies?
Tom: I don’t know, what?
Tim: A baseball team.
 
Google News Archive
5 July 1949, Lodi (CA) News-Sentinel, “Sports Chatter” by Chuck Driscoll, pg. 6, col. 2:
A publisher of children’s books must have had the Los Angeles and San Francisco Seals in mind when he inserted this riddle in one of his productions. “What has 18 legs and catches flies?”
 
Google News Archive
27 April 1965, The Free Lance-Star (Fredericksburg, VA), “Fun Time—The Riddle Box,” pg. 9, col. 2:
What has 18 legs and catches flies?
A baseball team.
 
Goole News Archive
28 April 1965, Milwaukee (WI) Sentinel, “Memorable Film Forgotten” by Buck Herzog, pt. 1, pg. 11, col. 1:
The salesmen, in order to make a hit with the kids, will be supplied with such jokes as, “What has 18 legs and catches flies? A baseball team,” etc.
     
Google Books
The Treasure-Trove of American Jewish Humor
By Henry D. Spalding
Middle Village, NY: Jonathan David Publishers
1976  
Pg. 285:
Bobby: Riddle me this, grandpa: What has 18 legs and catches flies?
Grandpa: Nine birds?
Bobby: No.
Grandpa: All right, so what has 18 legs and it’s catching flies?
Bobby: The New York Mets.
 
Google Books
Biggest Riddle Book in the World
By Joseph Rosenbloom
Illustrated by Joyce Behr
New York, NY: Sterling Publishing Company, Inc.
1976
Pg. 234:
What has 18 legs and catches flies?
A baseball team.
  .
Twitter
Diana Lees
‏@DianaLees
What has 18 legs and catches flies?  A Baseball Team! —Bennett Cerf
4:28 PM - 24 Apr 2009
 
Google Books
Riddles, Riddles, Riddles:
Enigmas and Anagrams, Puns and Puzzles, Quizzes & Conundrums
By Joseph Leeming
Illustrated by Shane Miller
Courier Corporation
2014
Pg. 128:
What is it that has eighteen legs and catches flies?
A baseball team.
 
reddit
Q: What has 18 legs and catches flies?
submitted June 20, 2016 by Chronochaotic
A: A baseball team